| http://www.w3.org/ns/prov#value | - The ply of sheet material is formed of a cushioning material such as leather, rubber,Y rubber composition, or synthetic rubber, while the tread ply g is formed of a sheet material having a requisite degree of hardness or surface finish to afford a desired traction characteristic; it being contemplated to provide a series of at least three tread inserts equipped with tread plys of diierent tractive
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